This invention relates to novel developments in the process described in our UK Pat. Nos. 1379513, 1380558 and 1385473 to obtain novel joints and end products.
We are concerned in this invention with the problems which arise in the establishment of electrical connections between conductors. At least in certain electrical and electronic fields known methods of forming connections (soldering using heat, mechanical plug-and-socket connections, clamping under pressure) are disadvantageous because they are potentially destructive or are unreliable.
One aspect of the invention concerns making permanent and strong electrically-conductive connections to electric circuit boards. Edge connections or terminal pads of such boards have either been soldered to plug sockets or have been directly mechanically engaged by contact sockets of a flexible conductor strip. Soldering and/or the flux used in soldering can damage the board substrate and/or adjacent elements on the board; mechanical engagement (whether direct or via a plug socket) is unreliable and liable to deteriorate in time.
The present process allows the formation of a permanent, strong, low resistance electrical connection without damage to the board or its pads since neither high pressure nor any sustained high temperature are used.
Any heat generated by the process is of such short duration and so highly localised at the interfaces of the materials to be joined that surrounding or adjacent areas of the components to be joined remain substantially unaffected and the substrates undamaged.
It also allows the soldering of components to a board whether at its edge or at other positions, without damage to the component or the board.
Another important aspect of the invention is forming in-situ welded connections between the outer conductors in cable joints over the cable core.
Most coaxial cables consist of a core conductor surrounded by an insulator which is often a thermoplastic, and which is in turn surrounded by the outer conductor or conductors. A further insulating or protective outer sheath completes the cable. Because of the nature of the inner insulating layer, it is very difficult by conventional means to produce welded or soldered joints in-situ between the outer conductors which do not severely damage the interior cable insulation of the cable at the joint (thereby distorting the cable).
The present process may be used to make permanent low resistance joints between the outer conductors of cables by welding a bridging connection which may be a strap between them without damage to the supporting insulating layers in the cables. The outer cable conductors which are to be joined may be braid screens or solid foils or a combination of braid and solid foil. They may be of aluminium, copper, tin coated copper, silver coated copper, zinc coated steel, tin coated steel, silver, or another metal or alloy.
If the connection is made by means of a bridging strap the bridging strap may be a strip or sleeve of braid or of solid metal, usually in foil form. It may be A1, Cu or Cu coated with Sn or Ag, Fe coated with Zn, Sn or Ag or another metal or alloy. It will not need cleaning before the welding operation and indeed may be coated with many non-metallic coatings without preventing the formation of strong metal-metal bonds with good conductive properties.
When a bridging strap is used it may be placed over, under or between the outer conductors of the cables.
The welds may be spot, seam or butt welds in any appropriate direction, e.g. longitudinal or circumferential.
Where overlap or butt welds are made there may be additionally present a brazing or soldering material which in the case of a butt weld is placed beneath the abutted edges and which will flow between the butted edges during the welding process without appreciable damage to the inner insulating material.
The process is characterised by placing the elements to be welded in contact and applying a high speed rotating frictional tool to the outer working surface opposite to the interfaces to be joined. This outermost working surface may be of a material to be welded or may be of an intervening material placed between the high speed tool and the outermost surface of the materials to be joined.
This is found, by a mechanism which is not understood, to generate instantaneously an extremely high energy level at the interface to form a weld.
In the case of similar metals, and whether the joint is an overlap or butt weld, the weld area may present a homogeneous crystal structure.
If dissimilar metals are comprised within the joint area considerable alloying and diffusion may occur. The joints created, however, are both mechanically strong and electrically sound with very low resistance values.
It would appear that the very high energy level generated at the interfaces is of such highly localised intensity and short duration that no heating of, or damage to, the elements or substrates surrounding or subjacent to the weld area occurs.
Joints need not comprise additional solder, brazing or filler materials of different composition and in the case of coated materials for example, thermoplastic coatings, the coating on at least one side of the foil or sheet remains substantially intact after the welding operation.
The actual joints per se and the products containing the joints of the invention therefore have novel characteristics.